1. Field of the Invention
The present invention relates to a surface acoustic wave device including a plurality of surface acoustic wave elements each of which includes a piezoelectric body, and more particularly, to a surface acoustic wave device utilizing, as a surface acoustic wave, a surface acoustic wave having a higher acoustic velocity than a bulk wave that propagates in the piezoelectric body.
2. Description of the Related Art
Surface acoustic wave devices have widely been used in a resonator and a band pass filter. The surface acoustic wave device can constitute resonators and filters in various frequency ranges depending on the types of surface acoustic waves utilized.
WO2003/088483 discloses a surface acoustic wave device utilizing a leaky surface acoustic wave among the plural types of surface acoustic waves.
Japanese Unexamined Patent Application Publication No. 8-65089 discloses a ladder filter using a surface acoustic wave resonator. In the disclosed ladder filter, an electric capacity is added to the surface acoustic wave resonator in series or in parallel. Japanese Unexamined Patent Application Publication No. 8-65089 states that a pass band width can be narrowed, for example, by adding the capacity.
On the other hand, WO2005/060094 discloses a boundary acoustic wave device utilizing a non-leaky propagation boundary acoustic wave. In WO2005/060094, a plurality of interdigital transducers (IDTs) are disposed on one piezoelectric single-crystal, and a propagation azimuth of an IDT is set to be different from that of at least another one IDT. As a result, according to WO2005/060094, a band width can be adjusted.
In the surface acoustic wave device utilizing the leaky surface acoustic wave, disclosed in WO2003/088483, it is also necessary to adjust the band width and resonance characteristics.
On the other hand, Japanese Unexamined Patent Application Publication No. 8-65089 states that, in the disclosed ladder filter, the band width can be adjusted by adding the capacity to the surface acoustic wave element in series or in parallel. However, such an arrangement has the problem that, because of the necessity of adding the capacity, the size of the surface acoustic wave device is increased and the cost is increased.
According to WO2005/060094, in the structure including a plurality of IDTs formed on one piezoelectric substrate, the propagation azimuth of at least one boundary acoustic wave element is set to be different from that of other boundary acoustic wave elements. The disclosed structure enables the band width to be adjusted without connecting an additional capacity.
However, the surface acoustic wave device utilizing the leaky surface acoustic wave, disclosed in WO2003/088483, has the problem that a propagation loss increases when propagation azimuths of plural surface acoustic wave elements are set to be different. The reason is that because the acoustic velocity of a leaky surface acoustic wave is higher than that of a slow transversal wave in the piezoelectric substrate, energy of the leaky surface acoustic wave leaks toward the piezoelectric substrate. In the surface acoustic wave device utilizing the leaky surface acoustic wave, therefore, sufficient or satisfactory filter characteristics and resonance characteristics cannot be obtained with the method of setting the propagation azimuths of plural surface acoustic wave elements to be different.